Mmm, you may struggle. @Paul-H (http://www.ngaugeforum.co.uk/SMFN/index.php?action=profile;u=6395) (IIRC) has done good things with TTS decoders including a totally reversible 37 conversion, ie no bits removed.

Obviously the sound is not as good as using a full size sugar cube but its still quite impressive.

Interestingly I have used this method of mounting the speaker on a number of sound decoders now

Class's 31, 37, 47, with TTS decoders and Class's 24 & 55 with Zimo decoders and they all have different sound qualities, some quite loud like the 37 and some quite quiet like the 55, but for me and my layout they are fine, mind you the Class 67 with TTS is fitted with a curved top sugar cube type speaker and its not that much louder, yet the 68 using the in built baffel is quite loud.

I sometimes think its more down to the settings of the recording rather than the speaker. I have a class 66 on a Zimo that is quiet regardless of which speaker I use.